Disposal of spent toners has long been a major problem for users of electrostatic plotters, printers and copiers. Environmental awareness, disposal costs, and strict governmental regulations relating to chemical handling and disposal have threatened the present manner of use of liquid toners. Purifying of toners presents a viable answer to these problems by reducing disposal amounts by allowing reuse of some of the toner. Presently, many prior attempts have been made to purify toners and other liquids.
In U.S. Pat. No. 5,457,485 to Moriyama et al., an ink jet recording apparatus is disclosed in which a screen filter is disposed across a flow of ink. Operationally associated with the screen is a movable baffle. The movable baffle periodically moves into abutting relationship with the screen filter. In this manner, a substantial portion of the screen may be blocked, thereby creating a large pressure differential between the opposed sides of the screen, allowing bubbles to pass therethrough. This design allows the screen to sequentially block and transmit bubbles, depending upon flow direction.
U.S. Pat. No. 5,426,459 to Kaplinsky discloses a thermal ink-jet pen cartridge having a check valve formed from a mesh having very small openings. The mesh openings are of sufficient size to prevent air bubbles from passing through under normal pressures. The check valve also serves to function as a particulate filter to prevent contamination of a printhead by particles from the ink reservoir.
U.S. Pat. No. 5,346,000 to Schlitt discloses a heat pipe equipped with a bubble trap. The bubble trap includes a baffle that restricts the liquid flow in a liquid flow channel of the heat pipe, as well as a wire mesh. The wire mesh is positioned downstream from the baffle to entrap bubbles.
U.S. Pat. Nos. 4,799,452; 4,895,103 and 4,923,581, all to G. F. Day, disclose methods for filtering liquid toner while eliminating the need for liquid toner disposal. In these methods, the toner itself is eliminated except for a transitory existence just at the moment of toning. Concentrated "ink" of each color is stored in a small tank and injected into and mixed with a continuous stream of clear dispersant. The resulting toner stream is passed through the toner applicator and then quickly decomposed back into concentrate and dispersant. This is achieved electrophoretically with a purifier, described in the above-referenced patents. The solid pigment particles are plated out on a rotating drum, then scraped off the drum and re-dispersed by vigorous mixing into the concentrate holding tank. To stop the toning process, the injection of the selected concentrate is simply terminated.
It would seem that this recycling concept might provide a liquid toning technology free of disposal problems since a large volume of contaminated or spent toner would never exist. However, the basic cause of disposal is not eliminated. Eventually, the contents of the concentrate tanks would have to be discarded due to contamination, as would the fluid in the dispersant tank. This is because the contaminants are re-mixed with the dispersant along with the pigment particles and are, therefore, never removed from the system. The quantity of liquid to be thrown away would be smaller, but some disposal problems would remain. The dispersant would have to be discarded when the conductivity level became high enough to interfere with image toning. A much higher level of contamination could be tolerated in the color concentrate tanks because of the dilution upon injection into the dispersant stream, but eventually the concentrate would also need replacement. In addition, the recycling architecture is relatively complex. It requires precise metering and mixing of two fluid streams and high speed separation of the toner into its components as it flows out of its applicator. With the high flows which are characteristic of full-width toner applicators, the separation apparatus must be quite large and, therefore, costly.
In order to electrophoretically separate a toner stream into its components, the fluid is passed between two closely spaced, parallel electrodes while a high voltage is imposed across the gap. All of the fluid must be exposed to the full electric field, and this means the flow must normally be confined to the gap region with some kind of fluid seals along the lateral edges of the separation zone. One of the electrodes should be moving so that the accumulating sludge can be scraped off and sent to the appropriate concentrate tank. The seals which confine the fluid flow within the gap present numerous technical difficulties.
Commonly assigned U.S. Pat. No. 5,404,210 describes an electrophoretic filtering method and apparatus which continuously purifies small portions of liquid toner by using a rotating drum type to keep contamination below the level at which it will interfere with imaging. Spaced-apart from the rotating drum is a conductive electrode with the drum and electrode defining a gap therebetween. Rotating drum-type purifiers are effective at removing solid contaminants such as particles of color pigment, paper debris, and ions from the fluid dispersant. This requires a high voltage potential being present across the gap. The apparatus would benefit from a higher level of purification for each pass, which could be achieved by increasing the voltage potential across the gap. However, electrical breakdown keeps the applied voltage lower than would otherwise be desired. The reduced voltage potential necessitates a very large purifier for any given flow rate to assure substantial separation of the solids from the clear fluid.
It is an object, therefore, of the present invention to provided an improved electrophoretic purifying system with an approach to reduce electrical breakdown between electrodes.
It is another object to provide a purification system which is of reduced size and increased efficiency.